Embolic protection is a concept of growing clinical importance directed at reducing the risk of embolic complications associated with interventional (i.e., transcatheter) and surgical procedures. In therapeutic vascular procedures, liberation of embolic debris (e.g., thrombus, clot, atheromatous plaque, etc.) can obstruct perfusion of the downstream vasculature, resulting in cellular ischemia and/or death. The therapeutic vascular procedures most commonly associated with adverse embolic complications include: carotid angioplasty with or without adjunctive stent placement and revascularization of degenerated saphenous vein grafts. Additionally, percutaneous transluminal coronary angioplasty (PTCA) with or without adjunctive stent placement, surgical coronary artery by-pass grafting, percutaneous renal artery revascularization, and endovascular aortic aneurysm repair have also been associated with complications attributable to atheromatous embolization. Intra-operative capture and removal of embolic debris, consequently, may improve patient outcomes by reducing the incidence of embolic complications.
The treatment of stenoses of the carotid bifurcation provides a good example of the emerging role of adjuvant embolic protection. Cerebrovascular stroke is a principle source of disability among adults, and is typically associated with stenoses of the carotid bifurcation. The current incidence of cerebrovascular stroke in Europe and the United States is about 200 per 100,000 population per annum (Bamford, Oxfordshire community stroke project. Incidence of stroke in Oxfordshire. First year's experience of a community stroke register. BMJ 287: 713-717, 1983; Robins, The national survey of stroke: the National Institute of Neurological and Communicative Disorders and Stroke. Office of Biometry and Field Studies Report. Chapter 4. Incidence. Stroke 12 (Suppl. 1): 1-57, 1981). Approximately half of the patients suffering ischemic stroke have carotid artery stenoses (Hankey, Investigation and imaging strategies in acute stroke and TIAs. Hospital Update 107-124, 1992). Controlled studies have shown that the surgical procedure carotid endarterectomy (CEA) can reduce the incidence of stroke in patients compared to medical therapy with minimal perioperative complications (<6% for symptomatic patients with stenoses >70% [NASCET, Beneficial effect of carotid endarterectomy in symptomatic patients with high grade stenoses. NEJM 325: 445-453, 1991] and <3% for asymptomatic patients with 60% stenoses [ACAS, Endarterectomy for asymptomatic carotid artery stenosis. JAMA 273: 1321-1461, 1995]). These results provide convincing evidence of the benefit of treating carotid stenoses. Surgery, however, does have several limitations, including: increased mortality in patients with significant coronary disease (18%), restriction to the cervical portion of the extra-cranial vasculature, a predeliction for cranial palsies (7.6%-27%), and restenosis (5%-19%; Yadav, Elective stenting of the extracranial carotid arteries. Circulation 95: 376-381, 1997).
Carotid angioplasty and stenting have been advocated as potential alternatives to CEA. Percutaneous techniques have the potential to be less traumatic, less expensive, viable in the non-cervical extracranial vasculature, and amenable to patients whom might otherwise be inoperable (Yadav, Elective stenting of the extracranial carotid arteries. Circulation 95: 376-381, 1997). Despite the potential benefits of this approach, emboli liberated during trans-catheter carotid intervention can place the patient at risk of stroke. Emboli can be generated during initial accessing of the lesion, balloon pre-dilatation of the stenosis, and/or during stent deployment. Additionally, prolapse of atheromatous material through the interstices of the stent can embolize after the completion of the procedure.
The fear of dislodging an embolus from an atherosclerotic plaque has tempered the application of angioplasty and endovascular stenting to the supraaortic arteries and, particularly, to the carotid bifurcation (Theron, New triple coaxial catheter system for carotid angioplasty with cerebral protection. AJNR 11: 869-874, 1990). This concern is warranted due to the significant morbidity and/or mortality that such an event might produce. While the incidence of stroke may be at an acceptable level for the highly skilled practitioner, it is likely to increase as the procedure is performed by less experienced clinicians.
Embolic protection devices typically act as an intervening barrier between the source of the clot or plaque and the downstream vasculature. In order to address the issue of distal embolization, numerous apparatus have been developed and numerous methods of embolic protection have been used adjunctively with percutaneous interventional procedures. These techniques, although varied, have a number of desirable features including: intraluminal delivery, flexibility, trackability, small delivery profile to allow crossing of stenotic lesions, dimensional compatibility with conventional interventional implements, ability to minimize flow perturbations, thromboresistance, conformability of the barrier to the entire luminal cross-section (even if irregular), and a means of safely removing the embolic filter and trapped particulates.
For example, occlusion balloon techniques have been taught by the prior art and involve devices in which blood flow to the vasculature distal to the lesion is blocked by the inflation of an occlusive balloon positioned downstream to the site of intervention. Following therapy, the intraluminal compartment between the lesion site and the occlusion balloon is aspirated to evacuate any thrombus or atheromatous debris that may have been liberated during the interventional procedure. These techniques are described in Theron, New triple coaxial catheter system for carotid angioplasty with cerebral protection. AJNR 11: 869-874, 1990, and Theron, Carotid artery stenosis: Treatment with protected balloon angioplasty and stent placement. Radiology 201: 627-636, 1996, and are commercially embodied in the PercuSurge Guardwire Plus™ Temporary Occlusion and Aspiration System (Medtronic AVE). The principle drawback of occlusion balloon techniques stem from the fact that during actuation distal blood flow is completely inhibited, which can result in ischemic pain, distal stasis/thrombosis, and difficulties with fluoroscopic visualization due to contrast wash-out through the treated vascular segment.
Another prior system combines a therapeutic catheter (e.g., angioplasty balloon) and integral distal embolic filter. By incorporating a porous filter or embolus barrier at the distal end of a catheter, such as an angioplasty balloon catheter, particulates dislodged during an interventional procedure can be trapped and removed by the same therapeutic device responsible for the embolization. One known device includes a collapsible filter device positioned distal to a dilating balloon on the end of the balloon catheter. The filter comprises a plurality of resilient ribs secured to the circumference of the catheter that extend axially toward the dilating balloon. Filter material is secured to and between the ribs. The filter deploys as a filter balloon is inflated to form a cup-shaped trap. The filter, however, does not necessarily seal around the interior vessel wall. Thus, particles can pass between the filter and the vessel wall. The device also presents a large profile during positioning and is difficult to construct.
The prior art has also provided systems that combine a guidewire and an embolic filter. The filters are incorporated directly into the distal end of a guidewire system for intravascular blood filtration. Given the current trends in both surgical and interventional practice, these devices are potentially the most versatile in their potential applications. These systems are typified by a filter frame that is attached to a guidewire that mechanically supports a porous filter element. The filter frame may include radially oriented struts, one or more circular hoops, or a pre-shaped basket configuration that deploys in the vessel. The filter element typically includes a polymeric mesh net, which is attached in whole or in part to the filter frame and/or guidewire. In operation, blood flowing through the vessel is forced through the mesh filter element thereby capturing embolic material in the filter.
Early devices of this type include a removable intravascular filter mounted on a hollow guidewire for entrapping and retaining emboli. The filter is deployable by manipulation of an actuating wire that extends from the filter into and through the hollow tube and out the proximal end. During positioning within a vessel, the filter material is not fully constrained so that, as the device is positioned through and past a clot, the filter material can potentially snag clot material creating freely floating emboli, prior to deployment.
In another prior art system an emboli capture device is mounted on the distal end of a guidewire. The filter material is coupled to a distal portion of the guidewire and is expanded across the lumen of a vessel by a fluid activated expandable member in communication with a lumen running the length of the guidewire. During positioning, as the device is passed through and beyond the clot, filter material may interact with the clot to produce emboli. This device may also be difficult to manufacture.
Another prior art device is adapted for deployment in a body vessel for collecting floating debris and emboli in a filter that includes a collapsible proximally tapered frame for operably supporting the filter between a collapsed insertion profile and an expanded deployment profile. The tapered collapsible frame includes a mouth that is sized to extend to the walls of the body vessel in the expanded deployed profile to seal the filter relative to the body vessel for collecting debris floating in the body vessel.
A further example of an embolic filter system involves a filter material fixed to cables or spines of a central guidewire. A movable core or fibers inside the guidewire can be utilized to transition the cables or spines from approximately parallel to the guidewire to approximately perpendicular to the guidewire. The filter, however, may not seal around the interior vessel wall. Thus, particles can pass between the filter and the entire vessel wall. This umbrella-type device is shallow when deployed so that, as it is being closed for removal, particles have the potential to escape.
Other disadvantages associated with the predicate devices are that the steerability of the guidewire may be altered as compared to the conventional guidewires due to the presence and size of the filter. The guidewire, for example, may bend, kink, and/or loop around in the vessel, making insertion of the filter through a complex vascular lesion difficult. Also, delivery of such devices in a low-profile pre-deployment configuration can be difficult. Further, some devices include complex and cumbersome actuation mechanisms. Also, retrieving such capture devices after they have captured emboli may be difficult. Further, when deployed in curved segments, the interaction of the guidewire and/or tether elements can deform the filter frame in such a way as to limit apposition to the host vessel wall, thereby allowing potential channels for passage of embolic debris. Also, the filter media of the prior art maintains a pore diameter of approximately 80 to 120 microns. It is desirable to minimize the pore size without adversely perturbing blood flow or being prone to clogging.
Current filter designs suffer from numerous disadvantages due to their construction. A typical wire filter is formed by manipulating multiple wires together through welding or some other form of attachment. After the wire frame is constructed, it is formed into the desired shape and a filter element is affixed onto the wire cage. A typical wire frame constructed in this manner is subject to a limited range of manipulation after the wires are adhered, since the welds or attachment areas are at an increased risk of failure due to the physical constraints of the welds themselves. A wire pair is more inclined to fracture at the weakest point, typically, a wire frame, composed of numerous wire pairs, will separate at the weld before separating in the length of the wire. Additionally, the welding of metal involves the application of increased heat to join a wire pair and a risk exists of the mesh, formed by the pairs, dripping or otherwise malforming due to the proclivity of metal to run before cooling.
A further disadvantage to a typical wire filter is that the filter element is difficult to apply to the frame since the filter is normally applied as a sock, tube, or other such shape. The typical wire frame is formed by welding and bending into the desired shape. The filter is then affixed onto the shaped wire frame by pulling the formed filter over the shaped wire frame. An additional problem evident in this construction is that the filter element could be abraded by a protrusion formed by a weld in a wire pair. Such an abrasion could form a weakness or a tear in the filter and undermine its desired functionality.
Simple and safe blood filtering and guidewire systems that can be temporarily placed in the vasculature to prevent distal embolization during endovascular procedures, and that can be used to introduce and/or exchange various instruments to a region of interest without compromising the position of the filter or guidewire, are required. Existing guidewire-based embolic filtering devices are inadequate for these and other purposes. The present apparatus, in contrast, provides a novel means of providing these and other functions, and has the further benefit of being easier to manufacture than the devices of the prior art.